EP0365988B1 - Developing solvent for layers which are crosslinkable by photopolymerization, and process for the production of relief printing forms - Google Patents

Description

The invention relates to a process for the production of relief printing plates crosslinked by photopolymerization, in which the layers that can be crosslinked by photopolymerization are exposed imagewise and the uncrosslinked portions of the layers are washed out with the developing solvent.

The production of relief shapes, in particular flexographic printing plates, by imagewise exposure and subsequent development and washing out of layers which can be crosslinked by photopolymerization has been known for a long time.

To produce such shapes, the photopolymer layer is exposed imagewise with actinic light; the formation of the relief is then made possible by washing out the unexposed and thus non-crosslinked portions of the layer with the development solvent. The task of the development solvent is to dissolve the non-crosslinked portions of the layer as quickly as possible, but on the other hand the solvent must be as easy to remove from the crosslinked portions of the plate so that the plate is quickly dry.

Therefore, development solvents with a low boiling point are mostly used today.

In DE-A 22 15 090 z. As methyl ethyl ketone, benzene, toluene, xylene, carbon tetrachloride, trichloroethane, trichlorethylene, methyl chloroform and tetrachloroethane and mixtures thereof are described as suitable solvents. A disadvantage of the chlorinated hydrocarbons mentioned is toxicity and disposal problems. If chlorinated hydrocarbons are used as development solvents, the areas to be removed swell very strongly on the plate, and the increasing concentration of the components of the non-crosslinked portions of the layer dissolved in the development solvent leads to a strong increase in the viscosity of the solvent. As a result, the capacity of the developing solvent is very limited and the solvent becomes unusable even with a 5% load. Another consequence of the strong increase in viscosity is that the plates can be developed very slowly even with a load of less than 5%. In addition, the development of plates containing nitrile rubbers as elastomers is very slow.

Of the non-chlorinated hydrocarbons mentioned in DE-A 22 15 090, benzene, toluene and xylene may be mentioned. These solvents have the disadvantage that they are easily flammable and also lead to a strong swelling of the layer and consequently only to a slow development and drying of the plate.

Because of their low flash point, they cannot be used in non-explosion-proof development devices.

DE-A 36 00 116 describes higher-boiling development solvents which contain, as essential constituents, branched, mono- or tri-olefinically unsaturated cyclic or saturated or mono- to tri-olefinically unsaturated cyclic aliphatic hydrocarbons, alcohols or ketones. Monoterpenes are mentioned in particular. The limonene used in the example has good development properties, but is sensitive to oxygen and irritating to the skin, properties that a development solvent should not have. Another disadvantage is that limonene only allows a very slow development and is difficult to remove from the layer. In addition, the development of plates containing nitrile rubbers is not possible within a reasonable time.

• nicht toxisch ist,
• einen hohen Flammpunkt besitzt,
• möglichst vollständig und schnell aus der Schicht entfernbar ist, ohne die Schicht zu quellen,
• sowie eine möglichst geringe Viskositätszunahme bei Aufnahme von Bestandteilen der Schicht aufweist, die es ermöglicht, trotz geforderter hoher Kapazität, schnell und einwandfrei entwickeln zu können,
• Platten, die Nitrilkautschuke als Elastomere enthalten, schnell entwickelt.

The object was therefore to provide a method for developing photopolymer layers with a development solvent, in which the solvent

• is not toxic,
• has a high flash point,
• can be removed from the layer as completely and quickly as possible without swelling the layer,
• and has the lowest possible increase in viscosity when absorbing components of the layer, which enables development to be carried out quickly and flawlessly despite the high capacity required,
• Sheets that contain nitrile rubbers as elastomers are quickly developed.

enthält, worin

R¹ bis R⁴

gleich oder verschieden sein können und H, (C₁-C₅)n- oder iso-Alkyl bedeuten, wobei R¹ und R², sofern sie direkt benachbart sind, auch einen cycloaliphatischen oder aromatischen Ring mit 5 oder 6 Ring-C-Atomen ausbilden können, die bevorzugt nicht substituiert sind,

mit der Maßgabe, daß die Summe der Kohlenstoffatome der Substituenten und der Ringglieder 9 bis 13 ist.The object is achieved by a process of the type described at the outset, which is characterized in that the development solvent is an essential component of an aromatic compound of the general formula I.

contains what

R¹ to R⁴

may be the same or different and denote H, (C₁-C₅) n- or iso-alkyl, where R¹ and R², if they are directly adjacent, can also form a cycloaliphatic or aromatic ring with 5 or 6 ring C atoms, which are preferably not substituted,

with the proviso that the sum of the carbon atoms of the substituents and the ring members is 9 to 13.

Aromatic compounds of the general formula I in which the number of carbon atoms is 9 to 11 (in particular 9 or 10) are particularly preferred.

Also preferred are those aromatic compounds of the general formula I in which the sum of the carbon atoms of the substituents is 3 to 5, provided that R 1 and R 2 are not closed to form a cycloaliphatic or cycloaromatic ring. In this case the sum of the carbon atoms of the remaining R is zero, i. H. R³ and R⁴ mean H.

Ring systems which can be formed by bridging the radicals R 1 and R 2 include indane, indene, naphthalene and tetralin. However, the C-9 ring systems indane and indene are preferred.

If the sum of the carbon atoms in the substituents is 5, this number 5 is preferably achieved by a single pentyl radical (for example R¹ = pentyl), while R² to R⁴ are H.

If the sum of the carbon atoms in the substituents is 3, this number is preferably generated by three methyl groups, a methyl group and an ethyl group or a propyl group.

If the sum of the carbon atoms in the substituents is 4, this number is replaced by four methyl groups, a methyl group and a propyl group, two ethyl groups, two methyl groups and an ethyl group or a butyl group.

The following aromatic compounds are particularly preferred:
Iso-propylbenzene, n-propylbenzene, 1-methyl-3-ethylbenzene, 1,3,5-trimethylbenzene, 1-methyl-2-ethylbenzene, 1,2,4-trimethylbenzene, isobutylbenzene, sec-butylbenzene, 1,2, 3-trimethylbenzene, 1-methyl-4-isopropylbenzene, indane, indene, 1,3-diethylbenzene, 1-methyl-4-propylbenzene, n-butylbenzene; 1-methyl-3-propylbenzene, 1,2,4,5-tetramethylbenzene, dimethylethylbenzene, methylindane, n-pentylbenzene.

The developing solvent can contain aromatic compounds of the general formula I both individually and as a mixture of various aromatic compounds which come under the general formula I.

The developing solvent used in the process according to the invention contains 55 to 100% by weight, in particular 55 to 90% by weight, of aromatic compounds of the general formula I.

In addition to the compounds of the general formula I, the developing solvent can contain other solvents as additives, especially if the polymer layer still contains a polyamide Contains protective layer. In this case, alcohols, in particular higher-boiling alcohols, are preferred as additional solvents.

The main constituents of the layers which can be crosslinked by means of photopolymerization and are to be developed with the development solvent essentially comprise a binder based on an elastomeric polymer, a photopolymerizable monomer which is compatible with the binder and a photoinitiator. The layers can also contain several different other binders, monomers or photoinitiators.

As additives, the layers can contain dyes, pigments, antihalation agents, antioxidants, plasticizers, antiozonants, crosslinking agents, regulators, fillers, fluxes and other agents which improve the effectiveness of the layer.

Other tools that can be added to the layer described are e.g. B. Thermal polymerization inhibitors such as hydroquinone and its derivatives, 2,6-di-tert-butyl-p-cresol, nitrophenols, nitrosamines such as N-nitrosodiphenylamine or salts of N-nitrosocyclohexylhydroxylamine, e.g. B. its alkali or aluminum salts.

Those layers which can be crosslinked by photopolymerization and which contain polymers of conjugated aliphatic dienes, their binders, are particularly preferred Have monomer units containing 4 to 5 carbon atoms. In particular, natural rubber, homopolymers of butadiene and isoprene, copolymers of butadiene and isoprene, copolymers of butadiene and / or isoprene with other monomers, such as styrene, vinyltoluene, acrylonitrile or (meth) acrylic acid alkyl esters, for. B. nitrile rubbers according to EP-A 064 564, random styrene / butadiene, - / isoprene, and - / isoprene / butadiene copolymers or block polymers of styrene monomers and butadiene and / or isoprene with a styrene content of 10 to 50% by weight %. Elastomers of this type are described in DE-B 22 15 090, DE-A 24 56 439, DE-A 29 42 183 and DE-A 21 38 582.

The layers which can be crosslinked by photopolymerization generally contain 20 to 95, preferably 30 to 95% by weight of binder.

Suitable monomers with one or more polymerizable olefinic double bonds are in particular esters and amides of acrylic and methacrylic acid. Examples are the compatible mono- and diacrylates and methacrylates of mono- or polyhydric alcohols, such as ethylene glycol, di-, tri-, tetra- or polyethylene glycols, the latter preferably with 10 to 15 ethylene glycol units, 1,3-propanediol, 1,6- Hexanediol, dodecanediol, glycerin, 1,1,1-trimethylolpropane, 1,2,4-butanetriol or pentaerythritol, e.g. B. ethylene glycol monomethacrylate, 1,3-propanediol monomethacrylate, hexanediol diacrylate, hexanediol dimethacrylate, 2-ethylhexyl acrylate, lauryl methacrylate, stearyl methacrylate, glycerol mono- or diacrylate, 1,2,4-butanetriol monomethacrylate, Pentaerythritol triacrylate, polyethylene glycol methyl ether acrylate, tetradecaethylene glycol dimethacrylate or the triether of glycerol and 3 mol N-methylolacrylamide or methacrylamide. The amount of monomers in the layer is generally from 1 to 70, preferably from 2 to 50,% by weight of the nonvolatile constituents of the mixture.

Suitable photoinitiators are the known compounds, which have sufficient thermal stability when processing the recording materials and sufficient radical formation when exposed to initiate the polymerization of the monomers. They are said to absorb light in the wavelength range from approx. 250 to 500 nm with the formation of radicals. Examples of suitable photoinitiators are acyloins and their derivatives such as benzoin, benzoin alkyl ethers, e.g. B. benzoin isopropyl ether, vicinal diketones and their derivatives, e.g. B. benzil, benzil acetals such as benzil dimethyl ketal, fluorenones, thioxanthones, polynuclear quinones, acridines and quinoxalines; further trichloromethyl-s-triazines, 2-halomethyl-4-vinyl-1,3,4-oxadiazole derivatives, halogen oxazoles substituted with trichloromethyl groups, carbonylmethylene heterocycles containing trihalomethyl groups according to DE-A 33 33 450, acylphosphine oxide compounds as described, for. B. are described in DE-A 31 33 419, and other phosphorus-containing photoinitiators, for example the 6-acyl- (6H) -dibenz- [c, e] [1,2] - described in German patent application P 38 27 735.2 oxaphosphorin-6-oxide, especially the 6- (2,4,6-trimethylbenzoyl) - (6H) dibenz- [c, e] [1,2] -oxaphosphorin-6-oxide.

The photoinitiators can also be used in combination with one another or with coinitiators or activators, e.g. B. with Michler's ketone and its derivatives or 2-alkyl-anthraquinones. The amount of photoinitiator is generally 0.01 to 10, preferably 0.5 to 5% by weight of the layer.

For the production of relief and flexographic printing plates, the mixtures which can be crosslinked by photopolymerization can be formed into layers with a thickness of 0.02 to 10, preferably 0.2 to 6 mm, by casting from solution or extrusion and calendering. The layer can be laminated to the surface of a suitable carrier or a solution of the mixture can be applied to a layer carrier.

In addition to the production of relief printing plates, the layers mentioned are also used for. B. used for the production of planographic printing plates, gravure cylinders, screen printing stencils and photoresists.

Suitable carriers are depending on the intended use, e.g. B. polyester films, steel or aluminum sheets, copper cylinders, screen printing stencil supports, foam layers, rubber-elastic supports or printed circuit boards. It may also be advantageous to have a cover or protective layer, e.g. B. a thin layer of polyvinyl alcohol or polyamide, or a peelable cover sheet, e.g. B. from polyethylene terephthalate. A pre-coating can also be used of the carrier may be advantageous. The additional layer between the support and the photosensitive layer may e.g. B. be effective as an antihalation layer or as an adhesive layer.

The layers which can be crosslinked by photopolymerization are exposed imagewise with actinic light from light sources such as mercury vapor lamps or fluorescent tubes, the emitted wavelength being 230 to 450 nm, preferably between 300 and 420 nm. The unexposed and uncrosslinked layer portions are removed by spraying, washing or brushing with the development solvent according to the invention. The developed relief molds are expediently dried at temperatures of up to 120 ° C. and, if necessary, post-exposed to actinic light simultaneously or thereafter.

The relief forms cross-linked by photopolymerization are particularly suitable for the production of printing forms, in particular letterpress and relief printing forms, which are particularly suitable for flexographic printing.

The invention is illustrated by the examples below.

example 1

A commercially available flexographic printing plate based on a three-block polymer made of styrene-isoprene-styrene as an elastomer (®Cyrel HL) with a layer thickness of 2.8 mm was first exposed over the entire surface for 76 s using a commercially available tube exposer and then imagewise through an applied negative original Exposed for 12 min from the front. The plate exposed in this way was then developed in a commercial, brushed processor with a mixture of aromatic hydrocarbons according to the general formula I of the composition listed below and a proportion of 15% by weight of butanol. The time to develop for optimal results was 5 min.

The flexographic printing plate was then dried at 60 ° C. for 2 hours and stored at room temperature for 15 hours. After the usual aftertreatment with an aqueous bromine solution, a flexographic printing plate of excellent quality was obtained.

Composition of the mixture of aromatic hydrocarbons:
2.7% mixture of n-propyl and iso-propylbenzene,
21.5% mixture of various isomeric methylethylbenzenes,
50.6% mixture of various isomeric trimethylbenzenes,
2.6% indan,
4.1% mixture of various isomeric dimethyl ethylbenzenes,
2.5% butylbenzene and
17% mixture of different alkylbenzenes with 9 to 11 carbon atoms.

Example 2

According to Example 1, a commercially available flexographic printing plate was exposed. The exposed plate was then developed in a commercial processor with the mixture of aromatic hydrocarbons from Example 1, but together with 15% by weight of 2-ethyl-butanol- (1). The time to develop for best results was 5 minutes. The flexographic printing plate was then post-treated as in Example 1. The result was a flexographic printing plate of excellent quality.

Example 3

A commercially available flexographic printing plate as in Example 1 was rubbed evenly into 50 ml of 1,3,5-trimethyltoluene for 5 minutes after removing the cover layer (plate size 18 × 3 cm). After drying and storage according to Example 1, a residual layer thickness of 1.28 mm was obtained.

The remaining layer thickness - in combination with the time available for development - is a measure of the development speed. The lower the The remaining layer thickness of the originally 2.8 mm thick layer is, the greater the development speed of the solvent used.

Example 4

The procedure was the same as in Example 3, with the exception that 4-isopropyltoluene was used as the developing solvent. The remaining layer thickness was 1.27 mm.

Example 5

The procedure of Example 3 was repeated, with the exception that a mixture of aliphatic-substituted aromatics, as described in Example 1, was used as the developing solvent. The residual layer thickness determined was 0.88 mm.

Example 6

The procedure was as in Example 5, with the exception that the solvent mixture from Example 2 was used. The remaining layer thickness was 0.4 mm.

Example 7 (comparative example)

In accordance with Example 3, a flexographic printing plate was exposed and developed, this time using limonene as the developing solvent. A residual layer thickness of 1.53 mm could be determined.

Example 8

A flexographic printing plate according to Example 1 was exposed over the full area from the rear for 76 s and then exposed for 8 min using a negative original from the front. After removing the cover layer, the plate (30 mm in diameter) was placed in 50 ml of mesitylene (1,3,5-trimethyltoluene) for 5 minutes. The plate was then wiped off, dried at 60 ° C. for 2 hours and stored for 17 hours. A weight gain of the layer of 1.97% could be determined.

Example 9

The procedure was as in Example 8, except that tetralin was used as the developing solvent. The weight gain in this case was 3.59%.

Example 10

The procedure was as described in Example 8, with the exception that the solvent mixture from Example 1 was used as the developing solvent. The weight gain was 2.7%.

Example 11 (comparative example)

The procedure was as described in Example 8, with the proviso that perchlorethylene was used as the developing solvent. The weight gain of the layer after development and drying was 4.3%.

Example 12

To determine the absorption capacity (capacity) of developing solvents on the components from non-crosslinked areas of a flexographic printing plate (®Cyrel HL), the viscosity of various developing solvents was used in the case of different solids contents of constituents from the layer after the development of a plate containing a three-block copolymer of styrene-isoprene-styrene as the elastomer.

Table 1 shows viscosity values, which were determined in the Ubbelohde viscometer at 25 ° C., for solids contents of 5, 7.5 and 10% by weight. While the development solvents according to the invention have a viscosity at a solids content of 7.5% by weight, which enables a very rapid development speed, the viscosity of perchlorethylene becomes so high even at a solids content of 5% by weight in the development solvent that a rapid development is no longer guaranteed. Table 1 Development solvent Viscosity [cSt] for different solids contents [% by weight] 5 7.5 10th 1,3,5-trimethylbenzene (mesitylene) 11.4 26.5 60 4-iso-propyl-4-toluene 15.8 35.4 76 Mixture from example 1 15 35.5 77 Perchlorethylene 37.1 120.8 333.5

Example 13

A commercially available flexographic printing plate (®Cyrel LP) based on nitrile rubber, which is suitable for printing with petrol inks, was treated as indicated in Example 6 for 15 minutes. The remaining layer thickness was 0.6 mm.

Example 14 (comparative example)

A commercially available flexographic printing plate (®Cyrel LP) was treated as indicated in Example 13. However, limonene was used as the developing solvent. The plate could not be developed.

Example 15 (comparative example)

A commercially available flexographic printing plate was treated as in Example 13. However, perchlorethylene was used as the developing solvent together with 15% by weight of n-butanol. The residual layer thickness was 1.21 mm after a development time of 15 minutes.

Claims (4)

1. Process for preparation of relief printing plates crosslinked by photopolymerization, wherein the layers crosslinkable by photopolymerization are exposed imagewise and the parts of the layers which are not crosslinked are washed out with the developer solvent, characterized in that the developer solvent contains as essential part an aromatic compound of the general formula I

   

   wherein

   R1 through R4   are equal or different and mean H, (C1-C5)n- or iso-alkyl, and wherein R1 and R2, when being immediate neighbors, can also form a cycloaliphatic or aromatic ring having 5 or 6 carbon atoms, which are preferentially unsubstituted,
   with the proviso that the sum of the carbon atoms of the substituents and of the members of the ring is 9 to 13.
2. Process according to claim 1, characterized in that the developer solvent contains 55 to 100 percent by weight, preferentially 55 to 90 percent by weight, of an aromatic compound of the general formula I.
3. Process according to claim 1 or 2, characterized in that the developer solvent contains in addition a higher boiling alcohol.
4. Process according to claim 1, 2 or 3, characterized in that the relief printing plate after wash-out is dried at temperatures up to 120 °C and is optionally exposed to actinic light either simultaneously or thereafter.